EasternTraveler

The official rule of credit cards:
"A credit card is not legally activated until signed." The signature validates the credit card like a signature validates a contract. Simple possession of the contract does not mean that you are legally bound to its terms nor that you agree to the terms of the contract. If you do not sign the card, you have in effect not agreed to the terms of the card as an instrument of payment and therefore are not agreeing to the terms of the card. From a contractual stand point, any merchant that accepts the card without it being signed is not making a complete agreement for future payment.

The supposed rule of credit cards:
The merchant should match the signature to the example on the back of the card. Doing so will prevent theft because the two should match. If a thief finds a card that is signed, they cannot sign it and also can not match the signature. If the card is not signed, the thief can now sign the card and the thieves signature of the name on the card will match the one on the card signature panel, because the thief signed it himself. This method only works when the merchant actually verifies the card. However, if only a few merchants check, there is still a small chance of a thief being caught.

I have had my signature panel checked multiple times. When a cashier asks for my ID, I refuse and say, "it is signed". They cannot legally demand my ID. Once they see it is signed and the signatures match, I am on my way.

I hope this is detailed enough and helpful.